Reflecting optical elements for use in the extreme ultraviolet spectral range (EUV), extending over wavelengths from about 10 nm to about 50 nm, can be implemented with multilayer mirrors, comprising as a rule a periodic sequence of layers containing a large number of thin-layer pairs. A thin-layer pair contains, generally speaking, two layers of different materials, and these should have the greatest possible difference in their optical constants in the range of wavelengths for which the component is intended to be used. At least one of these materials should exhibit the lowest possible absorption at the intended wavelength. The selection of the materials to be used in a multilayer mirror therefore depends primarily on the wavelength at which the optical component is intended to be used. In the EUV spectral range, therefore, there is an optimum pairing of materials for any particular wavelength range, usually only a few nanometers wide, where the optical contrast between the material layers guarantees high reflection.
For the wavelength range between about 12.5 and 14 nm, which is of great significance in particular for the development of optical systems for applications in EUV-lithography, multilayer mirrors using molybdenum and silicon as the material pair are preferred, as there is a particularly good optical contrast between these materials at the aforementioned wavelength range. Mo/Si (molybdenum/silicon) multilayer mirrors can, for instance, achieve a reflectivity of about 70% at a wavelength of 13.5 nm.
Reflectivity this high is of particular value for applications in which multiple reflections at multilayer mirrors takes place, since the reflectivity of the optical system as a whole falls in this case exponentially with the number of mirrors. In an arrangement of several mirrors, therefore, even a slight improvement in the reflectivity of a single mirror has a significant effect on the total reflectivity of the optical system. This is particularly true for optical systems used for EUV lithography, where, for instance, 11 multilayer mirrors may be used.
In order to achieve high reflectivity, it is particularly important to have the smoothest possible interfaces at the layer transitions between the molybdenum and silicon layers. On the other hand, however, the tendency of the molybdenum and silicon materials to form molybdenum silicide, MoSi2 in particular, and to undergo interdiffusion processes at the interface is known, for instance from DE 100 11 547 C2. There is therefore a risk, particularly at relatively high application temperatures, that multilayer mirrors of this type will degrade, as a result of which the reflectivity is significantly reduced. In addition to a loss of reflectivity, the degradation resulting from the interdiffusion processes and the formation of molybdenum silicide is also associated with a reduction in the thickness of the pair of layers, also known as the period thickness. This drop in the period thickness leads the maximum reflectivity to be shifted to a shorter wavelength. The function of an optical system based on multilayer mirrors of this type can be significantly impaired, or even fully destroyed, by degradation processes of this type.
Improving the thermal stability of Mo/Si multilayer mirrors by introducing a barrier layer of Mo2C between each of the interfaces between the molybdenum layer and the silicon layer is known from DE 100 11 547 C2.
Further, DE 100 11 548 C2 describes the use of barrier layers of MoSi2 to increase the thermal stability.
The systems of layers disclosed in the two publications mentioned above feature thermal stability, at least over a timescale of a few hours, up to 500° C. In comparison with a conventional Mo/Si multilayer mirror, however, they feature a comparably low reflectivity of less than 60%.
The introduction of barrier layers of B4C into Mo/Si multilayer mirrors in order to increase the reflectivity and/or the thermal stability is also known from U.S. Pat. No. 6,396,900 B1. While it is true that these layer systems are characterized by a comparatively high reflectivity of about 70%, the thermal stability, and in particular the long-term stability is not assured at temperatures of around 400° C. or above.
Laser plasma sources that emit radiation with a wavelength of about 13.5 nm are particularly used as the radiation source for the operation of optical systems applied to EUV lithography. Because the reflectivity of the optical system as a whole used in EUV lithography is relatively low as a result of the large number of mirrors, EUV radiation sources of this type must be operated at high powers to compensate for the reflection losses arising in the optical system. Close to high-powered EUV radiation sources of this type, EUV multilayer mirrors can be subjected to high temperatures. This is particularly true for an EUV multilayer mirror placed close to an EUV radiation source for the purposes of beam forming, for instance as what is known as the collector mirror.
There is therefore a need for multilayer mirrors that feature both high long-term thermal stability and high reflectivity.